The present invention is related to an oxide superconducting element and a superconducting device, and more particularly, to a superconducting element and a superconducting device for use in a high-speed circuit, a highly sensitive sensor, a current standard value determination and the like.
Superconducting tunneling is a phenomenon that appears by a quantum effect, and it is strongly considered for applying the quantum phenomenon to an electronic device. A tunnel-type Josephson element, which is one example thereof, is conventionally formed of a superconducting material such as Pb, Nb, NbN and the like. As a tunnel barrier, an oxide of the superconducting materials such as PbO and Nb2O3, or materials other than superconducting materials such as Al, A12O3, MgO, SiO2 and the like are used. The tunnel-type Josephson element being formed of such material show advantageous characteristics, and the application to superconducting memory, logic circuit, magnetic field sensor, and millimeter wave detection are promoted. However, such material systems show a low superconducting critical temperature below 15 K, and the cooling to a cryogenic temperature is necessary. The cost for performing such cooling is high, and the necessary equipment is large, so such superconducting materials could not be used easily. The characteristics of such materials under a weak magnetic field was studied and applied.
In the High Tc oxide superconductors, not only it is expected to reach a superconducting critical temperature Tc above the liquid nitrogen temperature, but it is also expected a phenomenon related to a physics of strong correlation or strong anisotropy. Therefore, studies aimed at tunnel-type elements are in progress. High Tc oxide superconductors have a layered crystal structure, with an intrinsic coupling characteristic being observed. From the point of view of relation to the superconducting structure, many studies are performed on the layered thin-film.
However, regarding the tunnel junction element utilizing an oxide superconducting material being manufactured artificially, reports are made only on the manufacturing of weak coupling-type elements having a difficult control characteristics, but no report is found so far regarding the tunnel-type element having an advantageous control characteristics. The reason for this is considered to be that the coupling between the superconducting electrodes are weakened since the manufacturing temperature of the oxide superconductor thin-film is above 700xc2x0 C., the mutual diffusion between the barrier layer causes deterioration, and the coherence length is short.
In order to realize a layered structure, it is advantageous to combine the same kind of material from a crystal growth point of view. Many results are reported where a PrBa2Cu3O7 having a similar crystal structure but showing no superconductivity is selected against a superconductor having a 123 structure, represented by a YBa2Cu3O7xe2x88x92d.
However, screw dislocations caused by strain were observed when using YBa2Cu3O7xe2x88x92d, and there was a problem in the surface flatness. Further, YBa2CU3O7xe2x88x92d does not have a good lattice matching performance with PrBa2Cu3O7. In oxide superconductors, the lattice strain is known to cause the increase of oxygen loss, and to deteriorate the superconducting characteristics. The Tc of an a-axis orientated film on a SrTiO3 single crystal substrate decreases to a lower value by the effect of the strain. The stress is stored in the interior of the multilayer film of YBa2Cu3O7xe2x88x92d and PrBa2Cu3O7, and makes it difficult to gain a high quality superconducting characteristics.
The inventors of the present invention have proposed an oxide superconducting Josephson element comprising a layered structure depositing an oxide superconducting thin-film and a non-superconducting thin-film layers alternately as the junction portion formed on the substrate (Japanese Patent Application Laid-Open No. 8-156627).
The oxide superconducting thin-film is a Mxe2x80x2Ba2Cu3O7 thin-film (where Mxe2x80x2 is an element of one or a combination of more then two rare earth elements such as Nd, Sm, and Eu), and the non-superconducting thin-film is a Mxe2x80x3Ba2Cu3O7 thin-film (where Mxe2x80x3 is either Pr or Sc, or a combination of these elements).
The analysis on the behavior of a superconducting element in a magnetic field including the above-mentioned oxide superconducting Josephson element was aimed mainly to the analysis of the behavior in a weak magnetic field, and almost no study was performed on the characteristics in a strong magnetic field.
It is well known that when an electromagnetic wave of a frequency f was applied to a Josephson element, a step appears in the current-voltage characteristics. The size of the voltage V of the step is proportional to the frequency f, and will be determined by the following relationship:
V=(h/2e)f
In the equation, h represents Planck""s constant, and e represents the charge of electron.
Therefore, when the frequency f is accurately known, then the voltage V could be calculated precisely. By utilizing such relations, a voltage standard could be made.
The present invention is based on the discovery that the oxide superconducting Josephson element proposed above showed a unique characteristics when analyzing the current-voltage characteristics in a high magnetic field. That is, by forming a layered structure having a small inner stress with a PrBa2Cu3O7 layer which is a non-superconductor positioned in the center, and a superconductor NdBa2Cu3O7 having a close lattice constant being selected to be deposited, the structure shows a voltage step generated at regular bias current intervals to the current-voltage characteristic curve in a magnetic field. The present invention is aimed to a superconducting device applying this voltage step.
The present invention relates to a superconducting regular current interval voltage step element: an oxide superconducting element having a layered structure which includes an oxide superconducting thin-film and a non-superconducting thin-film layer being deposited on a substrate. The current-voltage characteristic curve in a magnetic field includes a voltage step generated at regular bias current intervals.
Further, the layered structure is formed by alternately depositing a Mxe2x80x2Ba2Cu3O7 thin-film (where Mxe2x80x2 is one or a combination of more than two elements of Nd, Sm and Eu) and a Mxe2x80x3Ba2Cu3O7 thin-film (where Mxe2x80x3 is either Pr or Sc, or an element combining the two).
Furthermore, the present invention is related to a superconducting device comprising a superconducting element and utilizing a current-voltage characteristic showing a voltage step generated at regular bias current intervals in a magnetic field. The superconducting element is the above-mentioned superconducting regular current interval voltage step element.